Lyrids Meteor Shower 2015

UPDATE 24/04/15 Now that we’re past the peak it looks like the Lyrids meteor shower performed as expected. Reports from the Society for Popular Astronomy suggest that plenty of meteors were seen over the UK.

A wider survey made by volunteers submitting data to the International Meteor Organisation shows that a peak with ZHW=18 occurred more or less on cue around midnight on 22/23 April, with a possible second several hours later around 0700UT where the rate if anything was a little higher, with ZHR=22.

 

Over the next week one of spring’s best meteor showers will start to put on a show. The Lyrids meteor shower peaks overnight on the night of 22/23 April 2015, and should be best around midnight.

It’s quite hard to predict when exactly the peak will occur, and indeed you’ll still see some Lyrid meteors on the nights either side of the peak, so whenever you’ve got clear dark skies between now and 25 April it’s worth gazing skywards (isn’t it always?) in the hope that you’ll see a shooting star.

Why is the Lyrids Meteor Shower Happening This Week?

Meteor showers like the Lyrids happen when the Earth passes through a cloud of dust in space, These clouds are left behind by comets as they orbit the Sun, and the cometary cast-offs burn up in our atmosphere causing lots of bright streaks of light which we call meteors, or shooting stars. On any clear dark night you should see a few shooting stars, as random bits of space dust burn up overhead, but on the nights around the peak of a meteor shower, when the Earth is passing through a dense cloud of comet-dust, the rates can dramatically increase.

How Many Lyrids Will I See? There are a few ways you can maximise your chances of seeing some Lyrids (see The What, How, Where, When and Why of Meteor Showers) but the best way is to get somewhere dark, like one of the UK’s International Dark Sky Places. On the peak of the Lyrids meteor shower, under ideal conditions, you might see around 18 meteors per hour.

The peak of this particular shower doesn’t last very long, and so the rate on either side of the peak might be quite a bit less. Nonetheless it’ll still be well above the background rate of meteors. However the Lyrids occasionally surprises us and puts on a much better show. Back in 1982 there was a short-lived burst of Lyrid activity that saw the rate increase from 18 to 90. The same thing could happen this year: you never know until you look!

Ideal Conditions It’s the “ideal conditions” clause above that’ll reduce the rate from this maximum of 18. Ideal conditions are: perfectly clear skies; perfectly dark skies, free of light pollution; and the meteor shower radiant (the point where they all appear to emanate from) sitting directly overhead. The Lyrids’ radiant will be around 30° above the horizon at midnight, when the peak is meant to occur, but you can begin your meteorwatch as soon as it gets dark enough. You’ll then have until the sky brightens again pre-dawn. . The number of meteors that you will observe every hour depends on a number of factors:

• the density of the cloud of dust that the Earth is moving through, that is causing the shower in the first place;
• the height above the horizon of the radiant of the shower, the point from which the meteors appear to radiate;
• the fraction of your sky that is obscured by cloud;
• the naked-eye limiting magnitude of the sky, that is a measure of the faintest object you can see.

Crunching the Numbers The Lyrids meteor shower has a maximum zenith hourly rate (ZHR) of  around 18. This is the number of meteors that you can expect to see if the radiant is directly overhead (the point in the sky called the zenith), and you are observing under a cloudless sky with no trace of light pollution.

However conditions are rarely perfect. In the UK, for example, the radiant of the shower will not be at the zenith; it will be around 20° above the horizon at 2200, 30° above the horizon at 0000, 50° at 0200, to a maximum height of 70° pre-dawn.

Assuming a clear night, the other factor is the limiting magnitude of the sky, a measure of the faintest object you can see. Man-made light pollution will be an issue for most people. From suburbia the limiting magnitude of the sky is ~4.5 (around 500 stars visible), so you will only be able to see meteors that are at least this bright; the fainter ones wouldn’t be visible through the orange glow. In a big city centre your limiting magnitude might be ~3 (only around 50 stars visible); in a very dark site like Galloway Forest Dark Sky Park the limiting magnitude is ~6.5 (many thousands of stars visible), limited only by the sensitivity of your eye. So in most cases it’s best to try and get somewhere nice and dark, away from man-made light pollution.

The calculation that you need to make in order to determine your actual hourly rate is:

Actual Hourly Rate = (ZHR x sin(h))/((1/(1-k)) x 2^(6.5-m))

where h = the height of the radiant above the horizon

k = fraction of the sky covered in cloud

m = limiting magnitude

Let’s plug the numbers in for the Lyrids 2015.

ZHR = 18 (maximum) h = 30° at 0000 (assuming the maximum occurs at midnight; it might not) k = 0 (let’s hope!) m = 6.5 (if you get somewhere really dark)

So your actual hourly rate under clear dark skies is (18 x sin(30))/((1/(1-0) x 2^(6.5-6.5) = 9 meteors per hour If you’re observing in suburbia you need to divide this by around 4, and in bright cities by 10! Nonetheless, even in a city you’ll see a few Lyrids over the course of the night.

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Constellation of the Month: Leo the Lion

Head outside during April just as the sky gets properly dark and sitting high in the south is the constellation of Leo the Lion.

Leo is well-known as it’s one of the signs of the zodiac, and therefore one of the constellations through which the planets, Sun and Moon pass over the course of the year.

Leo is also well-known due to its most prominent feature, a pattern of stars within the constellation (called an asterism) known as The Sickle, which looks like a backwards question mark, with the bright star Regulus as the dot.

Regulus is known as the king star, and is one of the brightest stars in the sky, shining blue-white in late winter and spring.

Within the constellation of Leo are two groups of galaxies, marked as 1 and 2 on the chart above.

1. The Leo Triplet: M65, M66, and NGC3628

2. The M96 Group: Including M95 & M96

Any of these galaxies can be seen with even a small telescope, but their detailed structure can only really be seen in larger scopes.

Enjoy the spring skies, and happy galaxy hunting!

Maps and descriptions like this one for each of the 88 constellations can be found in my new book, Stargazing for Dummies. Click on the image on the right for more info.

The Lowest Full Moon of the Year

Tonight (actually around 0130 tomorrow morning) the Full Moon will reach its highest point due south, just an hour and a half after the eclipse ends. Despite being at its highest in the sky, you’ll still struggle to see it, as it is very low down. In fact the Full Moon nearest the Summer Solstice is the lowest Full Moon of the Year.

Full Moon by Luc Viatour http://www.lucnix.be

First, let’s begin with the definition of “Full Moon”. A Full Moon occurs when the Moon is diametrically opposite the Sun, as seen from the Earth. In this configuration, the entire lit hemisphere of the Moon’s surface is visible from Earth, which is what makes it “Full”. There is an actual instant of the exactly Full Moon, that is the exact instant that the Moon is directly opposite the Sun. Therefore when you see timings listed for the Full Moon they will usually include the exact time (hh:mm) that the Moon is 180° round from the Sun (we call this point opposition).

Here’s a list of the times of all Full Moons between June 2011 and June 2012:

Month	Date of Full Moon	Time of Full Moon (UT)
June 2011	15 June	2014*
July 2011	15 July	0640*
August 2011	13 August	1857*
September 2011	12 September	0927*
October 2011	12 October	0206*
November 2011	10 November	2016
December 2011	10 December	1436
January 2012	09 January	0730
February 2012	07 February	2154
March 2012	08 March	0939
April 2012	06 April	1919*
May 2012	06 May	0335*
June 2012	04 June	1112*

* UK observers should add on one hour for BST

As you can see from this table, the instant of the Full Moon can occur at any time of day, even in the daytime when the Moon is below the horizon. So most often when we see a “Full Moon” in the sky it is not exactly full, it is a little bit less than full, being a few hours ahead or behind the instant of the Full Moon. I’ll refer to this with “” marks, to distinguish this from the instant of the Full Moon (they look virtually identical in the sky).

The Moon rises and sets, like the Sun does, rising towards the east and setting towards the west, reaching its highest point due south around midnight (although not exactly at midnight, just like the Sun does not usually reach its highest point exactly at noon). And like with the Sun the maximum distance above the horizon of the “Full Moon” varies over the year.

The Sun is at its highest due south around noon on the Summer Solstice (20 or 21 June) and at its lowest due south around noon on the Winter Solstice (21 or 22 Dec) (of course the Sun is often lower than this, as it rises and sets, but we’re talking here about the lowest high point at mid-day, i.e. the day of the year in which, when the Sun is at its highest point that day, that height is lowest…)

And because Full Moons occur when the Moon is directly opposite the Sun, you can imagine the Moon and Sun as sitting on either sides of a celestial see-saw: on the day when the Sun is highest in the middle of the day (in Summer), the Moon is at its lowest high point at midnight; and on the day when the Sun is at its lowest high point in the middle of the day (in Winter), the Moon is at its highest high point at midnight.

This means, in practical terms, that Summer “Full Moons” are always very low on the horizon, while Winter “Full Moons” can be very high overhead.

Here’s a table of the altitude of the “Full Moon” when due south. Remember the times in this table don’t match the exact time of the Full Moon, but instead have been chosen as the closest in time to that instant, and so have be labelled “Full Moon” (in quotes).

Month	Date of Full Moon	Time of Full Moon (UT)	Time/Date of “Full Moon” due S	Time from/since instant of Full Moon	Altitude due S (degrees)**
June 2011	15 June	2014*	0127BST 16 June 2011	+4h13m	10° 05′
July 2011	15 July	0640*	0012BST 15 July 2011	-7h28m	10° 24′
August 2011	13 August	1857*	0126BST 14 August 2011	+5h27m	19° 19′
September 2011	12 September	0927*	0049BST 12 September 2011	-9h38m	31° 49′
October 2011	12 October	0206*	0053BST 12 October 2011	-1h13m	44° 16′
November 2011	10 November	2016	0005GMT 11 November 2011	-3h49m	53° 24′
December 2011	10 December	1436	0030GMT 11 December 2011	+9h54m	56° 03′
January 2012	09 January	0730	0006GMT 09 January 2012	-7h24m	53° 36′
February 2012	07 February	2154	0031GMT 08 February 2012	+2h37m	43° 47′
March 2012	08 March	0939	0000GMT 08 March 2012	-9h39m	35° 37′
April 2012	06 April	1919*	0145BST 07 April 2012	+5h26m	21° 45′
May 2012	06 May	0335*	0102BST 06 May 2012	-3h33m	15° 20′
June 2012	04 June	1112*	0047BST 04 June 2012	-11h25m	11° 49′

* UK observers should add on one hour for BST
** The altitude here is based on my observing location in Glasgow, Scotland. You can find out how to work out how high these altitudes are here.

As you can see from this table, the highest “Full Moon” due S this year occurs at 0030 on 11 December 2011, when the Moon will be over 56° above the southern horizon (approximately the height of the midsummer mid-day Sun which culminates at 57°34′).

Compare this to the “Full Moon” this month, just after the eclipse, in the morning of 16 June, when the Moon barely grazes 10° above the horizon, and you can see just how low the midsummer Full Moon can be.

In fact the closeness of summer “Full Moons” to the horizon means that this is an ideal time of year to try and observe the Moon Illusion.

UPDATE: Here’s a very cool speeded up video of the Moon cycling through its phases, as see by the LRO spacecraft: